Sometimes you want to take a site offline for a while. You can put this in .htaccess or the vhost config:

ErrorDocument 503 "We are performing maintenance on the site. Check back in a few minutes."
RewriteCond %{REMOTE_ADDR} !=
RewriteEngine On
RewriteRule .* - [R=503,L]

When using .htaccess, be sure to have AllowOverride All.

Or when using HTML files and images:

<VirtualHost *>
  DocumentRoot /var/www/down/
  ErrorDocument 503 /maintenance.html
  RewriteCond %{REQUEST_URI} =/maintenance.html [OR]
  RewriteCond %{REQUEST_URI} =/logo.jpg
  RewriteRule (.*) $1 [L]
  RewriteEngine On
  RewriteRule .* - [R=503,L]